Stored program controlled communication switching systems comprise some form of intelligence which controls switching functions in response to a program stored in memory. Historically, such systems included a single processing entity which controlled the entire communication path between subscribers to a telephone office. As technology and system design evolved, it was found desirable to separate certain routine functions from the main processing entity to save its processing time for more complex system functions and decisions. Systems, called distributed control systems, are presently being designed which also separate the control of portions of a communication path into several intelligent processors.
Before subscribers are connected to the communication path through a telephone switching system, checks should be performed to establish that the communication path is continuous. When all portions of the communication path are controlled by a single entity, such checks are relatively easy since all the necessary information is available in the single control entity. Such checks become quite difficult and time consuming when multiple control entities are used and each control entity must check what the others have done.
In one known switching system, a number of communication units communicate via time-multiplexed channels of a time-multiplexed switch. A central control, in response to routing information from an originating communication unit, establishes a communication path through the time-multiplexed switch and notifies both the originating and terminating communication units of the path identity. In the known system, both the originating and terminating communication units begin transmitting a logic one in a predetermined bit position of each data word transmitted on the established communication path between the originating and terminating communication units. Both the originating and terminating communication units monitor the predetermined bit positions for the presence of logic ones to verify the continuity of the path between them. However, in the typical case, many communication paths through the time-multiplexed switch are used for communication contemporaneously. With the known system, a terminating communication unit is unable to detect a time-multiplexed switch fault whereby data words are routed to the terminating communication unit from the wrong originating communication unit. Therefore, expensive self-checking hardware must be included in the time-multiplexed switch to detect such faults and a complex fault recovery procedure coordinated by the central control is required. In view of the foregoing, a recognized problem in the art is providing a switching system with the capability of detecting faults of the above-described type without adding undue cost and complexity to the system.